The purpose of cooling electronic equipment is to keep the temperature of the electronic components at some desired temperature. In some cases, in order to achieve the desired electronic performance, the electronic component must be cooled to a temperature below the temperature of its surroundings. In these cases, refrigeration equipment is used to pump the heat from the component into the surroundings. A typical electronic device requiring refrigeration is an infrared detector.
The Stirling cycle is a power-producing thermodynamic cycle using four basic thermodynamic processes. A Stirling cycle engine is basically a closed cycle system using regenerators to aid in the transfer of heat into and out of a working fluid. Although the Stirling cycle engine did not compete successfully with the later-developed steam and internal combustion engines, the Stirling cycle is still widely used in refrigeration.
When the motive power for a Stirling cycle refrigerator is a dc electric motor, it may be approximated that the amount of heat removed by the refrigerator is proportional to the voltage applied across the stator windings of the motor. Thus, in a closed-loop, temperature-controlled cooling system, the amplitude of the motor drive signal determines the amount of heat removed by the refrigeration apparatus and, therefore, the temperature of the cooled device.
In some cooling systems of this type, the motor is driven from a linear amplifier. Amplifiers are easily and efficiently compensated for supply voltage variations; however, in this type of system, a linear amplifier is highly dissipative, thereby contributing to inefficiency in the cooling system. Another form of motor drive uses pulse width modulation, which, for a sufficiently high pulse frequency, is very efficient in this type of cooling system.
In a typical pulse width modulation scheme, the duty cycle is controlled by an error signal generated from a temperature-sensitive transducer. However, because a dc motor is responsive to the average applied voltage for its rotational speed, it is easily seen that the pulse width modulation method depends upon a constant supply voltage. An increase in supply voltage will increase the average motor voltage, increasing the motor velocity, causing the temperature of the cooled device to drop. Although the inherent nature of the closed-loop system will eventually bring about self-correction, such temperature swings resulting from power supply variations are highly undesirable.
The effects of power supply variations can be reduced either by regulating the power supply or by compensating for the effects. The cost and complexity of regulating a power supply, particularly where it is employed as a power source for a large number of systems having current drains which vary over time, makes this an impractical option. It is therefore more practical to include in the cooling system an apparatus which compensates for the effect of power supply variations.